I too had that skipping problem on my Planet(s) on higher volumes and solved it by isolating the CDP with a slightly inflated 13" bicycle tube. I tied the tube in the middle to form and oval and it fit nicely under the unit. The feet now sit about a 1/2" off the self. On top of the unit and on either side of the CD trap door place some weight as described by Jgiacalo above. I used lead shot in those small zip-lock bags and then wraped them in black elastic stocking (an aesthetic thing). Have turned up volume and never had a recurring problem.
Good luck,
Lou
Good luck,
Lou